ISFETs fabrication method

ABSTRACT

Methods for fabricating ion sensitive field effect transistors (ISFETs) with SnO 2  extended gates. A SnO 2  detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO 2  detection film is electrically connected to a conductive wire, and an insulating layer is formed on the surface of the ISFET but part of the SnO 2  detection film and the conductive wire are left exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.

BACKGROUND

The invention relates to ion sensitive field effect transistors(ISFETs), and more particularly, to methods for fabricating SnO₂extended gate ISFETs.

The ion sensitive field effect transistor (ISFET) was presented by PietBergveld in 1970. An ISFET with reference electrode is similar toMetal-Oxide-Semiconductor Field Effect Transistor (MOSFET), except thatthe ISFET has exposed the gate insulator to measure a selected ionconcentration in electrolyte. When the pH-ISFET is immersed in anaqueous solution, a surface potential is induced at the surface of thedetection membrane of the pH-ISFET. However, the surface potential atthe sensing membrane will affect the carrier concentration within theinversion layer of the semiconductor, due to the gate dielectric layerbeing extremely thin. Thus, the current, which flows through thechannel, is adjusted. Furthermore, the surface potential is related tothe hydrogen ion activity within the aqueous solution. As the pH valueschange, different surface potentials are induced at the detectionmembrane, leading to different channel currents. Thus, the pH-ISFET canbe used to detect the pH values of solution.

Further, the extended gate field effect transistor (EGFET) structure waspresented by J. V. D. Spiegel et al, in which the detection film isextended from the gate terminal of the field effect transistor by aconductive line. Thus, only the detection film requires immersion in atesting solution, without the field effect transistor.

A variety of materials are known to be capable of serving as ISFETdetection film, such as, Al₂O₃, Si₃N₄, a-WO₃, a-C:H, and a-Si:H, etc.The manufacture of detection films is typically accomplished bydeposition, such as, sputtering or plasma enhanced chemical vapordeposition (PECVD). Thus, the cost is relatively high and the timerequired for thin film fabrication is excessive.

Thus, an easily fabricated, low cost ISFET and the detection filmthereof, eliminating packaging problems, are desirable.

SUMMARY

ISFET fabrication methods are provided. In an exemplary embodiment of afabrication method for ISFETs with SnO₂ extended gates, wherein a SnO₂detection film is formed on a substrate by sol-gel technology to serveas an extended gate. The SnO₂ detection film is then electricallyconnected with a conductive wire. An insulating layer is then formed onthe surface of the ISFET but leaving part of the SnO₂ detection film andpart of the conductive wire exposed. The exposed conductive wire iselectrically connected to a gate terminal of a MOS transistor.

In some embodiments of a detection circuit, a current mirror provides areference current, and a first operational amplifier comprises anon-inversion input terminal coupled to the reference current and aninversion input terminal coupled to a drain terminal of the ISFET. Afirst resistor comprises a first end coupled to the non-inversion inputterminal of the first operational amplifier, and a second end. A secondoperational amplifier comprises a non-inversion input terminal coupledto a source terminal of the ISFET, and an inversion input terminalcoupled to the second end of the first resistor. A drain-source voltagedetection module is coupled to the source terminal of the ISFET, and adrain-source current detection module is coupled to the source terminalof the ISFET.

In some embodiments of a read circuit, a detection circuit detects thedrain-source voltage and a drain-source current of the ISFET. Aprocessing unit is coupled to the detection circuit to determine a pHvalue of an unknown solution according to the detected drain-sourcevoltage and the detected drain-source current. A liquid crystal displayis coupled to the processing unit to display the determined pH value.

DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by the subsequent detaileddescription and examples with reference made to the accompanyingdrawings, wherein:

FIGS. 1A˜1D are flowcharts illustrating an embodiment of an ISFETfabrication method; and

FIGS. 2A and 2B are an exemplary embodiment of a read circuit.

DETAILED DESCRIPTION

FIGS. 1A˜1D are flowcharts illustrating an embodiment of a SnO₂ extendedgate ISFET fabrication method.

A substrate is cut into squares of 8 cm×8 cm and rinsed with propylalcohol, isopropanol and deionized water (DI water) respectively for 20minutes. A SnO₂ detection film 12 is then formed on the substrate 10 bysol-gel technology to serve as an extended gate, as shown in FIG. 1A.

In the step of forming SnO₂detection film 12, SnCl₂ powder (SnCl₂·2H₂O)is dissolved by ethanol to prepare a mixed solution with a concentrationof 0.37M. The mixed solution is then placed at room temperature for afirst predetermined duration, for example 3 days, to form a light yellowcolor mixed solution. The light yellow color mixed solution is coated onthe rinsed substrate 10, and the substrate 10 is dried at 350° C. in anoven for 1 hour to form the SnO₂ detection film 12, and then cooled toroom temperature.

The substrate 10 with the SnO₂ detection film 12 is cut into squares of1.5 cm×1.5 cm and washed in deionized water of an ultrasonic oscillator.One end of an aluminum conductive wire 14 is bonded to the SnO₂detection film 12 by silver paste and dried at 120° C. in an oven for 10minutes, and then cooled to room temperature, as shown in FIG. 1B.

The aluminum conductive wire 108 is installed through a capillary 16,and the SnO₂ detection film 12, the substrate 10 and the capillary 16are fixed by an insulating layer 18 comprising epoxy resin and dried at120° C. in an oven for 20 minutes. The SnO₂ detection film 12 and thesubstrate 10 are then packaged with epoxy resin but an area of 2 mm×2 mmis kept to serve as a sensing window. The sensing portion 19 is obtainedas shown in FIG. 1C.

Finally, the remaining end of the aluminum conductive wire 14 iselectrically connected to a gate of a MOSFET 20, and the ISFET 100 witha SnO₂ detection film 12 is completed.

In order to read out pH values of unknown solutions a read circuit isalso provided to detect a drain-source voltage and a drain-sourcecurrent of the ISFET 100 to thereby determine pH values of unknownsolutions accordingly. During pH value detection, only the detectionfilm is required to be immersed into the unknown solutions without theMOSFET connected to the detection film. FIGS. 2A and 2B show anexemplary embodiment of a read circuit. Read circuit 200 includes adetection circuit 120, a processing unit 130, a liquid crystal display140, a reference voltage generation module 150 and an oscillation signalgeneration module 160.

The detection circuit 120 detects the drain-source voltage V_(DS) anddrain-source current I_(DS) of an extended gate ISFET (called EGFEThereinafter) 100. The detection circuit 120 includes a current mirror121, a first operational amplifier A₁, a resistor R₂, a secondoperational amplifier A₂, a drain-source voltage detection module 123and a drain-source current detection module 125.

The current mirror 121 includes three bipolar junction transistors Q₁˜Q₃and a resistor R₂, and provides a reference current I_(REF) for thedetection circuit 120. The transistor Q₁ includes a first terminalcoupled to a power voltage V_(DD), a second terminal coupled to anon-inversion input terminal of the operational amplifier A₁, and acontrol terminal coupled to a control terminal of the transistor Q₂. Thetransistor Q₂ includes a first terminal coupled to the power voltageV_(DD), a second terminal coupled to the resistor R₁, and a controlterminal coupled to the control terminal of the transistor Q₁.

The transistor Q₃ includes a first terminal coupled to the controlterminals of the transistors Q₁ and Q₂, a second terminal coupled to aground voltage, and a control terminal coupled to the resistor R₂ andthe second terminal of the transistor Q₂. The resistor R₁ includes afirst end coupled to the ground voltage and a second end coupled to asecond terminal of the transistor Q₁ and the control terminal of thetransistor Q₂. For example, the resistor R₂ can be a variable resistorto adjust the drain-source voltage V_(DS) within 0.0˜1.0V, and thecurrent mirror 121 is a constant current source. Further, the currentmirror 121 and the resistor R2 combine a constant voltage source togenerate a required voltage V_(DS) to control the EGFET 100. The firstand second operational amplifiers A₁ and A₂ constitute a source followerto prevent a loading effect, and the desired voltage V_(D) at the drainterminal of the EGFET 100 is obtained by the source voltage V_(S) andthe loop constituted by the operational amplifiers A₁ and A₂ and theresistor R₂.

The operational amplifier A₁ includes a non-inversion input terminalcoupled to second terminal of the operational amplifier A₂ and theresistor R₂, an output terminal coupled to the drain terminal of thetransistor 20, and an inversion input terminal coupled to the outputterminal thereof. The operational amplifier A₂ includes a non-inversioninput terminal coupled to the drain terminal of the transistor 20, anoutput terminal coupled to the resistor R₂, and an inversion terminalcoupled to the output terminal thereof.

The drain-source voltage detection module 123 is coupled to the sourceterminal of the EGFET 100 to detect the drain-source voltage V_(DS) ofthe EGFET 100. As shown, the drain-source voltage detection module 123includes an operational amplifier A₃ and resistors R₆˜R₉. For example,the resistors R₆˜R₉ can be the same, such that the output voltage of theoperational amplifier is equal to V_(D)˜V_(S), namely, the drain-sourcevoltage V_(DS). The resistor R₆ includes a first end coupled to thenon-inversion input terminal of the operational amplifier A₃ and theresistor R₇, and a second end coupled to the output terminal of theoperational amplifier A₁ and the drain terminal of the transistor 20.The resistor R8 includes a first end coupled to the non-inversion inputterminal of the operational amplifier A₂ and the source terminal of thetransistor 20, and a second end coupled to the inversion input terminalof the operational amplifier A₃ and the resistor R₉. The resistor R₉ iscoupled between the second end of the resistor R₈ and the outputterminal of the operational amplifier A₃.

The drain-source current detection module 125 is coupled to the EGFET100 to detect the drain-source current of the EGFET 100. Thedrain-source current detection module 125 includes a Widlar currentsource, an operational amplifier A4 and a resistor R₅. The Wildarcurrent source includes four transistors Q₄˜Q₇ and two resistors R₃ andR₄ to control and limit the drain current I_(D). The resistor R₄ can bea variable resistor to adjust the current I_(DS), and the transistor Q₇,the resistor R₅ and the operational amplifier A₄ constitute asub-detection unit to detect the current I_(DS).

The transistor Q₄ includes a first terminal coupled to the groundvoltage, a second terminal coupled to the control terminal of thetransistor Q₂ and a control terminal coupled to a first end of theresistor R₃, and a second end of the resistor R₃ is coupled to theground voltage. Transistor Q₅ includes a first terminal coupled to thefirst end of the resistor R₃ and the control terminal of the transistorQ₄, a control terminal coupled to the control terminal of the transistorQ₆, and a second terminal coupled to the power voltage Vss. Thetransistor Q₆ includes a first terminal coupled source terminal of thetransistor 20, a control terminal coupled to control terminal of thetransistor Q5, and a second terminal coupled to a first end of thefourth resistor R₄, and a second end of the fourth resistor R₄ iscoupled to the power voltage Vss. The transistor Q₇ includes a firstterminal coupled to the first end of the resistor R₅, a control terminalcoupled to the control terminal of the transistor Q₄, and a secondterminal coupled to the first end of the resistor R₄. The operationalamplifier A₄ includes a non-inversion input terminal coupled to thefirst end of the resistor R₅ and the first terminal of the transistorQ₁, and an inversion input terminal coupled to an output terminalthereof. The output terminals of the operational amplifiers A₃ and A₄and the source terminal of the transistor 20 are coupled to the inputterminals of the processing unit 130 respectively.

The reference voltage generation module 150 includes two operationalamplifiers A₅ and A₆, two zener diodes Z₁ and Z₂ and resistors R₁₀ andR₁₁. The zener diodes Z₁ and Z₂ provide stable voltages to theprocessing unit 130 through the operational amplifiers A₅ and A₆, suchthat the processing unit 130 can potentially prevent source noise andreduce fluctuation errors.

The oscillation signal generation module 160 includes an oscillator OCSand two capacitors C₁ and C₂ to provide oscillation signals to theprocessing unit 130.

The processing unit 130 determines the pH value of unknown solutionsaccording to the detected drain-source voltage V_(DS) and the detecteddrain-source current I_(DS) via detection circuit 120. The processingunit 130 can be microprocessor PIC16F873 manufactured by Microchip. Theliquid crystal display 140 is coupled to the processing unit 130 todisplay the determined pH value of unknown solutions.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A fabrication method for ion sensitive field effect transistors(ISFETs) with a SnO₂ extended gate, comprising: providing a substrate;forming a SnO₂ detection film on the substrate by sol-gel technology toserve as an extended gate; electrically connecting the SnO₂ detectionfilm with a conductive wire; forming an insulating layer on the surfaceof the ISFET but exposing part of the SnO₂ detection film and part ofthe conductive wire; and electrically connecting the exposed conductivewire and a gate terminal of a MOS transistor.
 2. The fabrication methodas claimed in claim 1, wherein the insulating layer comprises epoxyresin.
 3. The fabrication method as claimed in claim 1, wherein formingthe SnO₂ detection film on the substrate comprises: dissolving SnCl₂powder by ethanol to form a mixed solution; placing the mixed solutionfor a first predetermined duration to form a light yellow color mixedsolution; coating the light yellow color mixed solution on thesubstrate; and heating the substrate to a first predeterminedtemperature for a second predetermined duration.
 4. The fabricationmethod as claimed in claim 3, wherein the concentration of the solutioncomprising SnCl₂ and ethanol is about 0.37M.
 5. The fabrication methodas claimed in claim 4, wherein the first predetermined temperature is350° C.
 6. The fabrication method as claimed in claim 4, wherein thesecond predetermined duration is 1 hour.
 7. The fabrication method asclaimed in claim 1, further comprising rinsing the substrate for a thirdpredetermined duration by propyl alcohol, isopropanol and deionizationwater (DI water) respectively before forming the SnO₂ detection film. 8.The fabrication method as claimed in claim 7, wherein thirdpredetermined duration is 20 minutes.
 9. The fabrication method asclaimed in claim 2, wherein connecting the SnO₂ detection film onsubstrate with the conductive wire comprises: rinsing the substrate withthe SnO₂ detection film by DI water; fixing a first terminal of theconductive wire to the SnO₂ detection film by silver paste; and heatingthe substrate to a second predetermined temperature for a fourthpredetermined duration.
 10. The fabrication method as claimed in claim9, wherein the fourth predetermined duration is 10 minutes.
 11. Thefabrication method as claimed in claim 9, wherein forming an insulatinglayer on the surface of the ISFET comprises: installing the conductivewire through a capillary; packaging the substrate, the SnO₂ detectionfilm, the conductive wire and capillary by the insulating layer butexposing part of the SnO₂ detection film, part of the conductive wireand part of the capillary; and heating the substrate to the secondpredetermined temperature for a fifth predetermined duration.
 12. Thefabrication method as claimed in claim 11, wherein the secondpredetermined temperature is 120° C.
 13. The fabrication method asclaimed in claim 11, wherein the fifth predetermined duration is 20minutes.